iPX opens and reads without a connection (#389)

With no network the home-screen app showed nothing: no service worker,
and the lists the page had fetched, full text included, lived only in
its memory.

A service worker (web/sw/sw.ts, TypeScript checked against a worker's
globals by tsconfig.sw.json, built by swc to /sw.js) goes to the network
first for the page and every API read and keeps the answers, so online
nothing is an old copy and #388's version check works as before;
offline the kept copy answers. Scripts named by hash come from what is
kept. Reads, flags and places in an episode that cannot reach the
server wait in an outbox, sent in order when the connection is back
(and new ones queue behind them meanwhile, so none lands out of turn);
a kept list served offline shows the reads waiting there.

Pictures: a worker cannot keep another site's pictures sensibly, since
browsers count each opaque answer as megabytes. ipx now remembers the
picture addresses in item text it sends (bounded, in memory) and
/api/art fetches those as it does artwork, through the guarded client;
the worker keeps them, for unread items, under a 200 MB budget, and
serves them offline in place of the publisher's. Online, pictures still
come from their publisher.

Sign-out, or another account signing in on the device, clears what was
kept. The browser suite blocks the worker except in the offline tests,
since it answers before page.route can.

The fixture JPEGs are placeholders a browser cannot draw; the offline
test uses a real PNG.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/CHANGELOG.md

@@ -13,6 +13,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
+- iPX opens and reads without a connection: the lists it has fetched (All Subscriptions, Unread and the feeds it preloads or you opened) are kept on the device with the pictures in their unread items, up to 200 MB of them, and items read or flagged and places in an episode while offline are sent when the connection is back. Signing out clears it all (#389).
 - An open page, or the home-screen app, takes up a new version of iPX by itself once it reconnects after a deploy, back on the item you had open; while something is playing, a dialog is open or a search is typed, it offers a reload instead (#388).
 
 ### Fixed